Widely used manufacturing process is transfer metallizing. This process employs the stages of coating a carrier film, metallizing the coating, laminating the coated, metallized carrier film to a substrate, and finally removing the carrier film from the substrate. Such a transfer metallizing process is described in U.S. Pat. No. 4,344,998. The substrate is adhered to the coated, metallized carrier film by means of a so-called dry adhesive, that is an adhesive which had been dissolved in an organic solvent.
After application of this adhesive, the solvent has to be evaporated by means of drying ovens. Subsequently, the dried adhesive is brought into contact with the substrate, is then allowed to cure in such a way that finally, after the adhesive is hard enough, the carrier film can be removed.
This known process has several disadvantages. First of all, the dried adhesive is not very mobile. This means that it cannot provide good penetration and thereby adherence to a rough surface, such as uncoated paper or board, because it is difficult to penetrate such irregularly shaped surface due to its very high viscosity after evaporation of the solvent.
Moreover the adhesive takes time to harden before the carrier film can be separated from the product. Only after such time has elapsed, is it possible to check whether the bond between substrate and metal layer is strong enough. This of course represents an economic disadvantage.
It is also known to use a so-called wet adhesive, that is a water based adhesive. Such a process is described in GB-A-2006109. The water based adhesive is applied to the coated, metallized carrier film and subsequently the substrate is laminated without drying the adhesive. The substrate must have an absorbent character, such as board or paper, which absorbs the water from the adhesive.
This process has the advantage that the adhesive is hard as soon as the water is absorbed by the substrate. Thereby, the carrier film can be removed even in-line, directly after laminating the substrate. Moreover, relatively cheap uncoated and/or recycled papers and boards can be used in this process, thus giving high quality products with high internal strength at high production speeds, and at low cost.
The disadvantage of this process however is that it cannot provide very good barrier properties, in particular with respect to gasses. First of all, the rough, uncoated surface of the papers and boards used causes the metal layer to become distorted and broken up. Moreover, the papers and boards, due to the composition of cellulose fibers, are greatly affected by changes in humidity. As the humidity increases, the paper or board expands; as the humidity decreases, they contract. As a result, the metal layer is caused to break leading to a loss of barrier properties.